1. Field of the Invention
This invention relates to measurement of the performance of integrated circuit fabrication tools and, more particularly, to a method for measuring performance of cluster tools.
2. Description of the Relevant Art
An article titled "Cluster Tool Performance Tracking" (by Vallabh H. Dhudshia and Clyde Hepner, of Texas Instruments, Inc., "Future Fab International", Issue 1, Volume 1, Technology Publishing, Ltd., 1996) stated, "The world of semiconductor processing has continued to evolve toward more complex manufacturing equipment and systems. Single chamber, single process equipment has been progressively replaced by multi-chamber systems." Multi-chamber tools are also referred to as "cluster tools."
The Semiconductor Equipment and Materials International (SEMI) E10-96 standard Guideline for Definition and Measurement of Equipment Reliability, Availability, and Maintainability (RAM) is hereinafter referred to as "E10" or the "E10 standard" and is incorporated by reference in its entirety. The E10 standard broadly defines a cluster tool as "a manufacturing system made up of integrated processing modules mechanically linked together (the modules may or may not come from the same supplier)." Examples of cluster tool modules include process chambers, transfer robots, vacuum pumps, and gas systems.
A paper "Managing Multi-chamber Tool Productivity" (by Bruce Aches, Gulsher Gewal, and Peter Silverman, of the Intel Corporation, Santa Clara, Calif., published at the "1995 IEEE/SEMI Advanced Semiconductor Manufacturing Conference") states, "Multi-chamber tools have enabled significant technical breakthroughs in wafer processing in the last decade. Expenditures for these tools are increasing as a percentage of the total capital base. Fully one-third of the capital dollars invested in 0.4 micron technology processing equipment will be spent on multi-chamber tools." The paper goes on to state, "By transferring wafers between process steps through a vacuum load-lock, multi-chamber tools significantly reduce contamination as compared to systems that allow exposure to air during transfer for many applications. For this and other reasons, highly integrated process applications such as multi-layer metal deposition, clean/deposition and deposition/etch sequences using multi-chamber tool technology are becoming more pervasive."
Broadly speaking, cluster tools may be classified as either "sequential" or "parallel" tools. Sequential tools are also commonly referred to as "serial" tools. A serial tool processes a wafer in a series of sub-steps performed in different modules of the serial tool. The sub-steps are commonly referred to as a "recipe." Therefore, when one module of a sequential tool is "down", i.e., inoperable to perform its sub-step, the entire tool cannot produce any product by a recipe requiring that a sub-step to be performed by the inoperable tool. Only when all of the modules of the sequential tool required by a given recipe are "up", i.e., operable to perform their designated process sub-step or operation, is the tool capable of producing the desired product according to the recipe. However, it is common for a process tool to be capable of performing multiple recipes. Hence, a recipe which does not require the down module may still be run and produce the desired product. Product is defined as any unit which is intended to include a functional device. A unit is defined as any wafer, die, packaged device, or piece part thereof. Examples of processing operations performed by process chamber modules are metal deposition, oxide growth, and source/drain implant.
In contrast to sequential cluster tools, when one or more modules of a parallel cluster tool are inoperable, the remaining modules of the tool may or may not be capable of continuing to produce product, albeit at a reduced rate, i.e., with reduced performance. The Auches paper states, "It is misleading to consider the tool up to production when in reality it cannot produce at a full run rate. It is equally misleading to consider the tool completely down when the tool can produce wafers, although at a reduced run rate."
Cluster tools are typically relatively expensive pieces of equipment. The Auches paper states that the most sophisticated cluster tools on the market today cost approximately between one million and two million dollars. Accurately determining the performance of these expensive cluster tools may enable a company to make more informed decisions about whether to procure, or to postpone procuring, additional processing tools in order to meet production demands. Therefore, it is desirable to accurately determine the performance of cluster tools.
Various measures of the performance of equipment have been developed over the past few decades. An example of these performance measures is the SEMI E10-96 standard, which defines states and formulae for determining the reliability, availability, maintainability, and utilization of process tools.
Another example of an industry accepted performance measure is the overall equipment effectiveness (OEE) metric. The OEE metric defines formulae based on the availability, operational efficiency, rate efficiency, and rate of quality for computing the effectiveness of process tools. These performance measures have been defined and effectively applied to single chamber integrated circuit fabrication tools.
A limitation of the E10 and OEE performance measures is that the states and metrics defined by them are directly applicable only to single chamber tools. This limitation is due mainly to the complexity of cluster tool operations. The following example with reference to FIG. 1 examines the availability of a cluster tool and illustrates the problems associated with applying currently defined performance measurement standards to cluster tools. Availability relates to the time which a process tool is available to perform wafer fabrication operations relative to the total time over which the performance is to be measured.
The cluster tool 8 from Novellus.RTM. Systems, Inc. of FIG. 1 can be defined as having five modules: two process chambers 8A and 8B, two loadlock chambers 8D and 8E, and a transfer robot 8C for transferring wafer cassettes between the loadlock chambers 8D and 8E and the process chambers 8A and 8B. The cluster tool 8 is inoperable as a whole if the transfer robot 8C is down, if both of the process chambers 8A and 8B are down, or if both of the loadlock chambers 8D and 8E are down. If one of the process chambers 8A or 8B is down, or if one of the loadlock chambers 8D or 8E is down, or if one process chamber and one loadlock chamber are down, the cluster tool 8 is still operable, however, at a reduced performance relative to a configuration where all of the modules are operable.
In a scenario where the transfer robot 8C is up for an entire week, i.e., the availability of the transfer robot 8C is 100% for 168 hours, but both of the process chambers 8A and 8B are up for only 100 hours and down the other 68 hours, the question arises, "what is the availability of the cluster tool as a whole over the 168 hour period?" The E10 and OEE standards do not currently provide an answer to such a question. Therefore, an improved method for determining the performance of cluster tools is desired.